This invention relates to electro-magnetic wave absorbing material.
Such material is used for preventing ghosts of iron towers, ships, etc. in radar systems, and for preventing generation of unwanted electric wave and dispersion thereof.
As disclosed in Japanese Laid Open Patent Specification No. 36987/1980, a prior art electro-magnetic wave absorbing material was constituted by a mixture of ferrite, carbon black and a high molecular weight synthetic resin. Such material absorbs electro-magnetic wave energy by the magnetic loss caused by ferrite and electric loss caused by carbon black. Accordingly such electro-magnetic wave absorbing material is coated or deposited on the surface of a metal or electro-conductive sheet to a specific thickness so as to form an electro-magnetic wave absorber.
The capability of absorbing electric wave of such absorber, however, differs depending upon the frequency and intensity of the electric wave to be absorbed so that it has been desired to provide improved electro-magnetic wave absorbing material or member having a high absorbing efficiency.
In order to increase the loss of the electro-magnetic wave imparted by the absorbing material described above, it is necessary to increase the quantities of incorporation of ferrite and carbon. However, even when the quantity of ferrite is increased there is a limit for the magnetic loss, while carbon black can increase electric loss, its function becomes unstable due to variations in electric conductivity and particle size.
As is well known in the art, the electro-magnetic wave absorption capability of the electric wave absorbing member described above has the maximum absorption efficiency when the input impedance Z as measured from the surface of a layer of the electro-magnetic wave absorption material is unity, where ##EQU1## where .epsilon..sub.2 represents a complex dielectric constant of the absorption layer, .mu..sub.2 a complex permeability, .lambda. wavelength and d the thickness of the absorption layer.
However, the frequency bandwidth in which a condition Z=1 holds is very narrow. For this reason, it has been difficult to obtain satisfactory electro-magnetic wave absorption characteristic over a wide frequency bandwidth.